Recording materials that employ color development through a reaction between a color former and a color-developing agent allow recording in a short time using a relatively simple apparatus without performing complicated treatments such as development and fixation and are thus widely used in thermal recording paper for output recording in facsimiles, printers, etc., or pressure-sensitive copying paper or the like for forms for simultaneous multiple copying. These recording materials are required to immediately develop colors, maintain the whiteness of an uncolored part (hereinafter, referred to as a “background”), and offer high colorfastness of colored images. Particularly, recording materials excellent in the heat resistance of a background and images are desired in terms of long-term storage stability. For this purpose, attempts have been made to develop color former, color-developing agents, storage stabilizers, etc. Nevertheless, recording materials that have well-balanced, sufficiently satisfactory color-developing sensitivity, background and image stabilities, etc., have not been found yet.
Besides, as recording materials excellent in the background stability, 4-hydroxy-4′-isopropoxydiphenylsulfone, etc. are conventionally known, but their heat resistance is not still satisfactory.
On the other hand, although a phenol color-developing agent such as 4,4′-isopropylidene diphenol shows good color-developing performance, such an agent can correspond to an endocrine disruptor and is not allowed to be used for some users. Therefore, a color-developing agent having a structure not containing a phenol skeleton (which structure is hereinafter referred to as the non-phenol) is required.
Patent Document 1 describes a non-phenol color-developing agent. A synthesis method for this color-developing agent is, however, complicated, and in addition, this agent is inferior in plasticizer resistance. Patent Document 2 describes a sulfonamide color-developing agent. Although such a color-developing agent is good in the color developing performance, one having a sulfonyloxy phenyl group is poor in water resistance, and in addition, if a sulfonyloxy group having a hydrolyzable property is decomposed, a phenol compound is produced.
Therefore, the present inventors have found that a compound having structures of both N-phenyl benzenesulfonamide and carbanilide can be used as a color-developing agent. As the compound having such a structure, sulfonamide compounds described in Patent Document 3 are known, but these compounds are used for preventing infection with microorganisms such as bacteria, and use as a color-developing agent is not disclosed.